Effects of surfactant on carbon nanotube assembly synthesizedby direct spinning

Junyoung Song a, Sora Yoon a, Soyoung Kim a, Daehwan Cho b,nn, Youngjin Jeong a,n

a Department of Organic Materials and Fiber Engineering, Soongsil University, Seoul 156-743, South Koreab School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA

H I G H L I G H T S

� In this study, a surfactant was added to the synthesis solution of CNTs to reduce the aggregation of catalysts.� It was found that the addition of surfactant significantly influenced the crystalline quality, diameter and number of walls of CNTs.� It was noted that the catalysts dispersed uniformly during the CNT synthesis increased their reactivity and then enhanced the properties of CNTs.

a r t i c l e i n f o

Article history:Received 21 June 2013Received in revised form26 August 2013Accepted 2 September 2013Available online 2 September 2013

Keywords:Carbon nanotubeSurfactantCVD SynthesisCatalystAgglomeration

a b s t r a c t

A surfactant is used to reduce the agglomeration of iron catalysts during the synthesis of carbonnanotubes (CNTs) and their assembly by chemical vapor deposition. The agglomeration of iron catalystsduring synthesis affects the CNT diameter and number of CNT walls. Also, the agglomerated catalystswithin the CNT assembly indicate an inefficient reaction of catalysts during the CNT synthesis. In thisstudy, the non-ionic surfactant, polysorbate, is added to the CNT synthesis solution in varying amountsfrom 0 to 3.0 wt%. The effects of the surfactant are closely related to the crystalline perfection, diameter,and number of walls of CNTs synthesized under different concentrations of the surfactant. Here, 1.0 wt%surfactant is the most favorable concentration to achieve crystalline perfection. When the surfactant isadded at more than 1.0 wt%, amorphous carbons increase, although the agglomeration of iron catalystscontinues to decrease. The simple addition of this surfactant can facilitate the development of the CNTassemblies that have high mechanical and electrical properties.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon nanotubes (CNTs) have been studied in a wide range ofapplications since the discovery of CNTs was reported in Nature byIijima in 1991, because of their exceptional thermal, electrical, andphysical properties, and their high aspect ratio (Iijima, 1991;Basiuk and Basiuk, 2008; De Heer et al., 1995). These CNTs aremostly synthesized in powder form, which make them difficult todisperse at high concentrations or to maintain their concentra-tions at high levels, due to van der Waals' forces, or cohesiveforces. These properties have been a barrier to the application ofCNTs in many areas (Thess et al., 1996; Ausman et al., 2000;Dervaux et al., 2012). For that reason, recent studies have focusedon the fabrication of CNT assemblies in which the outstandingproperties of CNTs can be fully manifested (Baughman et al., 2002;

Vigolo et al., 2000, 2002; Zhang et al., 2007, 2004; Lynch andPatrick, 2002; Li et al., 2004). For example, Windle (2004)proposed direct spinning, which is a technique for the synthesisof CNTs and their assembly resulting in its continuous production.A liquid carbon source and catalyst with a carrier gas are injectedinto a high-temperature vertical furnace to synthesize CNTs. Thesynthesized CNTs take aggregate forms, and at the bottom of thefurnace the CNT aggregates are wound up into web- or fiber-likeCNT assembly.

Since the introduction of direct spinning by Li et al. (2004),studies have described the effect of process conditions, such as thecomposition of a solution and the temperature of a synthesisfurnace, on the diameters of CNTs (Lu et al., 2012; Zu et al., 2012),or the densification of CNT assembly by acetone or water (Koziolet al., 2007). Windle's group focused on how the composition of asolution and other process conditions affected CNT diameters,because the CNT diameter is substantially related to the propertiesof CNTs, such as the numbers of CNT walls and band gaps. Further,because the CNT diameter is directly affected by the size of catalystparticles during synthesis, their study examined how the solution

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0009-2509/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.ces.2013.09.008

n Corresponding author. Tel.: þ82 2 820 0667.nn Corresponding author.E-mail addresses: dc575@cornell.edu (D. Cho), yjeong@ssu.ac.kr (Y. Jeong).

Chemical Engineering Science 104 (2013) 25–31

composition and other process conditions are related to the sizeand agglomeration of catalyst particles. Later, Windle reportedthat when a catalyst precursor was added in excess, it remained inthe form of an agglomerated catalyst in a CNT assembly. Tocompensate the agglomerated problem, he adopted carbon dis-ulfide (CS2) as a promoter, instead of thiophene and methane gas

as a carbon source (Sundaram et al., 2011). The reason why CS2was selected that it decomposes at 600–750 1C, approximately200 1C lower than the decomposition temperature of thiophene at800–1000 1C. Furthermore, the use of CS2 decreases the agglom-eration of iron particles emitted from the catalyst precursorferrocene by reducing the time it takes for them to be transformedinto active catalysts and, as a result, causes small-diameter CNTs tobe formed.

These findings indicate that the physical properties of CNTs aregreatly affected by the agglomeration and size of catalysts. Ingeneral, agglomeration of iron catalysts occurs when the ferrocenedissociates above 500 1C under H2 (Leonhardt et al., 2006).Additionally, the irons tend to agglomerate until they form iron-sulfide. The sulfide is supplied from thiophene decomposition at800–1000 1C. Thus, the present study investigated a new methodfor reducing the agglomeration of iron catalysts during thesynthesis process. In doing so, we added surfactant to a CNTsynthesis solution and explored the effect of this addition on theagglomeration of catalysts. In addition, we examined whatchanges took place in the properties of CNT assemblies as the sizeof agglomerated catalysts got smaller.

2. Experimental

2.1. Materials

The acetone (99.7%) used as a carbon source was purchasedfrom Samchun Chemical (Korea). Ferrocene (Z98%) was used as acatalyst precursor, thiophene (Z99%) as an activating agent, andsodium lignosulfonate, sodium dodecyl sulfate and polysorbate_20as surfactants, all of which were commercially obtained fromAldrich.

2.2. Synthesis of carbon nanotubes

In this study, the CNT synthesis solution was prepared bymixing acetone 96.0–99.0 wt%, ferrocene 0.2 wt%, thiophene

Fig. 1. Synthesis of CNTs and their assembly by direct spinning.

Fig. 2. The changes in dispersed surfactant over time and their absorbance plotted asa function of time. (a) Change after 0.5 min, (b) change after 20 min, (c) absorbancegraph: anionic surfactants (① sodium lignosulfonate [SLS],② sodium dodecyl sulfate[SDS]), ③ cationic surfactant (cetyltrimethylammonium bromide [CTBA]), and ④

nonionic surfactant (Polysorbate_20 and its molecular formula).

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0.8 wt% and surfactant 0.0–3.0 wt%. As shown in Fig. 1, theprepared solution was injected with hydrogen gas into the furnaceheated to 1200 1C at the top of the reactor. Iron particles wereemitted when ferrocene was decomposed by thermal energyabove 500 1C in the reactor (Leonhardt et al., 2006), and iron-sulfide was formed as sulfur was emitted from thiophene, whichserved as an active agent (Li et al., 2004). Then, the iron-sulfide

formed a liquid phase, and carbon nanotubes grew on the ironcatalysts from when the carbon supplied by the decomposition ofacetone was saturated by diffusing into the iron-sulfide. If thesolution was continuously supplied into the synthesis furnace, theCNT assembly continued to form (Fig. 1). The principles of thissynthesis were described in detail by Li et al. (2004) and Zhonget al. (2010). However, it is very difficult to trace the location

Fig. 3. (a) SEM image of the CNT assembly synthesized by direct spinning and (b) contents of elements by position in the CNT assembly.

Fig. 4. Catalyst agglomeration in the CNT assemblies with varying amounts of polysorbate addition.

J. Song et al. / Chemical Engineering Science 104 (2013) 25–31 27

where the agglomeration starts because gas flow rate and tem-perature in the reactor are subject to change in radial and lengthdirections (Conroy et al., 2010). In this study, the synthesis solutionwas injected at a rate of 10 ml/h with a hydrogen gas flow rate of1000 sccm, and the synthesized CNT assemblies were wound up inthe range of 7.5 m/min at the bottom of the furnace.

2.3. Characterization

The Raman spectra of synthesized CNT assemblies were mea-sured using Horiba's LabRAM HR polarized Raman instrumentwith a radiation wavelength of 514 nm and an objective lens of�50 magnifications, and the assemblies were exposed to the laser

for 20 s. To observe the CNT structure that was synthesized undereach condition, SEM images were obtained using JEM-3010HRTEM (JEOL), and the catalyst content in the CNT fiber wasmeasured with Energy Dispersive Spectroscopy (EDS, EX-250model of Horiba). The SEM images were also analyzed using imageanalysis software (Motic Image Plus 2.0) to quantify the degree ofcatalyst agglomeration. The agglomeration of iron catalysts in CNTassemblies and the changes in their physical properties wereexamined using a constant composition of the CNT synthesissolution, but with the addition of varying amounts of the nonionicsurfactant polysorbate.

3. Results and discussion

3.1. Selection of surfactants

Surfactants were selected on the basis of their stability in theacetone used as a carbon source. The changes in dispersedsurfactant were observed over time after their addition to theCNT synthesis solution (Fig. 2). The changes in dispersed surfactantwere determined by observing the UV–visible transmittance andprecipitation as a function of time. The most stable solution wasobtained with the use of the nonionic surfactant, polysorbate_20.Accordingly, polysorbate_20 was adopted as the surfactant forthis study.

3.2. Effect of surfactants on catalyst agglomeration

Each CNT assembly synthesized by direct spinning had whiteagglomerates formed in it (Fig. 3(a)). To identify their components,the areas of white agglomerates were distinguished from the CNT

Concentration of polysorbate (wt.%)0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Are

a oc

cupi

ed b

y ag

glom

erat

ed c

atal

ysts

(%)

0

2

4

6

8

10

12

14

Fig. 5. Percent of area in CNT assemblies occupied by agglomerated catalysts atdifferent concentrations of added polysorbates.

Fig. 6. Agglomeration of iron catalysts in CNT assemblies with (1.0 wt%) and without (0 wt%) of polysorbate. FE-SEM images are on the left, and the corresponding SEM-based EBSD images where only the parts with iron catalysts are highlighted are on the right.

J. Song et al. / Chemical Engineering Science 104 (2013) 25–3128

assembly and they were analyzed. Fig. 3(b) shows the contents ofcarbon and iron in each of the two areas. Iron existed at a highercontent in the white aggregated area than in the other. Therefore,iron is the main component of the agglomerates. A small concen-tration of iron was found in the non-white area as well, due to theiron catalysts involved in the growth of CNTs. The observation ofoxygen suggests that the iron catalysts were present in the form ofiron oxide. This catalyst agglomeration, which is commonlyobserved in CNT assemblies synthesized by direct spinning,indicates that the catalysts were not efficiently involved in the

synthesis and they prevented CNT assemblies from developingtheir inherent properties.

Fig. 4 shows the FE-SEM images of CNT assemblies prepared atdifferent concentrations of polysorbate. The amount of ironcatalyst particles agglomerated in the assembly was reduced withthe increase in the polysorbate concentration. Fig. 5 shows thepercent areas of agglomerated catalysts at different polysorbateconcentrations, which were obtained from SEM image analysis ofFig. 4. Non-agglomerated catalysts were not well observed on theSEM images because they were very small and presented inside

Fig. 7. HR-TEM images of samples without the addition of polysorbate (a, c and e) and with the addition of 1.0 wt% polysorbate (b, d and f). Arrows indicate amorphouscarbons. Insets in (c) and (d) are magnified images.

J. Song et al. / Chemical Engineering Science 104 (2013) 25–31 29

CNTs. This indicates that when the percent area occupied byagglomerated catalysts was lower, the dispersion of the catalystswas better. The largest area of agglomeration was observed whenpolysorbate was not added, and the total area of agglomeratedcatalysts was reduced with the addition of polysorbate, showing arapid decrease until 1.0 wt% polysorbate was added. Therefore, itappears that the agglomeration of catalysts during the synthesis ofCNT assemblies could be effectively reduced by the addition ofpolysorbate.

To clarify the effect of polysorbate, the samples were measuredusing electron back-scattering diffraction (EBSD) at a highermagnification. ESBD is a type of X-ray technique that acquiresimages of crystals using the laser light reflected when the emittedlaser beam passes through a sample (Hurley and Humphreys,2003). This technique is also used to assay different components inalloys or assemblies. On EBSD images, elements with higheratomic numbers appear brighter than elements of lower atomicnumbers. In this study, this technique was used to analyze the ironcatalysts and carbon nanotubes in CNT assemblies. A CNT assemblyis mainly composed of carbon and iron, and thus the area of ironcatalysts appears brighter on its images. Fig. 6 shows the EBSDimages of agglomerated iron catalysts in CNT assemblies with andwithout the addition of polysorbate. The area of iron catalystparticles appeared concentrated on the EBSD images where thesurfactant had not been added, whereas the iron particlesappeared dispersed in the images where 1.0 wt% surfactant hadbeen added.

3.3. Effect of polysorbate addition on CNT morphology

Fig. 7 shows HR-TEM images to identify what changes theaddition of a surfactant made in the morphology of CNTs. Therewas a clear difference between the CNT samples with and withoutthe addition of 1.0 wt% polysorbate. When the polysorbate was notadded, catalyst particles were found in many places, but when1.0 wt% polysorbate was added, catalyst particles were not easyto detect in CNTs. The CNTs synthesized by the addition ofpolysorbate had very clean morphologies, and catalysts and

amorphous carbon were rarely observed in them. EDS results inTable 1 also indicates that agglomerated iron catalysts wereexposed to the surface of CNT assembly. This suggests that theaddition of polysorbate induced the effective dispersion of ironcatalysts and thus the synthesis of CNTs occurred with minimalagglomeration.

The insets in Fig. 7(c) and (d) are HR-TEM images in which thenumber of CNT walls can be counted with and without theaddition of polysorbate. We observed 3–5 walls in CNT sampleswithout the addition of polysorbate and 2–3 walls with theaddition of 1.0 wt% polysorbate. These observations are consistentwith the results of the preceding section that the agglomeration ofiron catalysts was reduced due to the presence of polysorbate and,as reported in previous studies (Gohier et al., 2008; Lee et al.,2012), small-diameter CNTs had a smaller number of walls (Fig. 8).

Fig. 8 shows how the CNT diameter varies depending on theamount of polysorbate addition. The average diameter of CNTs wasreduced with the addition of polysorbate, which indicates that theaddition of polysorbate contributed to reducing the size of ironcatalysts. Thus, polysorbate facilitated the formation of small-sizeiron catalysts by interfering with the agglomeration of ironparticles and, accordingly, enabled the synthesis of small-diameter CNTs with a small number of walls. The change in CNTdiameter due to the addition of polysorbate occurred only up to asurfactant concentration of 1.0 wt%, and the diameter remainedalmost unchanged at higher concentrations. Our results suggestthat there is a critical point beyond which the surfactant cannotreduce the agglomeration of catalysts. Such a critical concentrationis likely to vary depending on the type of surfactant, and thisshould be verified by further studies.

3.4. Effects of polysorbate addition on crystalline perfection

Raman spectroscopy was used to analyze the effects of addedpolysorbate on the crystalline perfection of CNTs. Fig. 9(a) showsthe Raman spectra of CNT fibers synthesized with the addition ofthe nonionic surfactant polysorbate while varying the concentra-tion conditions between 0 and 3.0 wt%. The addition of polysor-bate did not affect the position of Raman peaks, but the increase inpolysorbate concentration led to changes in the intensity ratio of Gand D bands (IG/ID). The G-band (near 1580 cm�1) indicatedfeatures of graphite, but the D-band (near 1350 cm�1) suggestedthat there were disordered features of graphitic sheets (Chenget al., 1998). Thus, a higher value of IG/ID ratio indicated that theCNTs featured fewer defects and a higher crystallinity (Thomsenand Reich, 2000). As shown in Fig. 9(b), the IG/ID ratio wassignificantly affected by the addition of a surfactant. The reasonfor such changes can be inferred from the HR-TEM images in Fig. 7.On the image of the sample with no polysorbate added to it (Fig. 7(a)), amorphous carbon is seen commonly in CNTs, whereas on theimage of the sample with 1.0 wt% polysorbate added to it (Fig. 7(b)), there are only very small amounts of amorphous carbonaround CNTs. Apparently, the agglomeration of catalysts wasreduced due to the addition of polysorbate; the effective reactionof the catalysts during the synthesis of CNTs produced lessamorphous carbon. It is known that iron cannot work as a catalystif it exceeds a certain size, in which case more amorphous carbonis produced during the synthesis of CNTs (Li et al., 2007). In otherwords, if the agglomeration of catalysts is reduced by polysorbate,it leads to the production of active catalysts in larger numbers andamorphous carbon in smaller amounts, and the IG/ID valuebecomes high with the growth of crystalline CNTs. However, thecrystalline perfection was reduced at higher than 1 wt% as shownin Fig. 9(b). This effect occurred because the excessive amount ofpolysorbate increased amorphous carbon in CNT assemblies.

Table 1Contents of elements in the CNT assemblies with and without the addition ofpolysorbate (the data are the average values of 3 samples).

Content of polysorbate added to synthesissolution (wt%)

Carboncontent (%)

Ironcontent (%)

0 80.9 19.11 84.8 15.2

Fig. 8. Average diameter of CNTs at different concentration of polysorbate.

J. Song et al. / Chemical Engineering Science 104 (2013) 25–3130

4. Conclusion

This study investigated the effects of the surfactant addition onthe characteristics of CNT assemblies fabricated by direct spinning.In doing so, we synthesized CNT assemblies with the additionof the highly-soluble nonionic surfactant polysorbate to a CNTsynthesis solution at varying concentrations. The morphology,crystalline perfection, diameter, and number of walls of CNTassemblies and the agglomeration degree of iron catalysts weremeasured and analyzed with increasing polysorbate concentra-tions from 0 to 3 wt%. In CNT assemblies synthesized without theaddition of polysorbate, agglomerated catalysts were observed inlarge amounts, but the agglomeration of iron catalysts decreasedas the concentration of added polysorbate increased. Crystallineperfection showed a peak at a polysorbate concentration, but thecrystalline perfection was reduced at higher concentration levels.This effect occurred because excessive polysorbate increased theamount of amorphous carbon in CNT assemblies although thepolysorbate caused small-diameter CNTs to be synthesized withhigh crystalline perfection by interfering with the agglomerationof catalysts.

Our results suggest that the addition of a surfactant improvedthe qualities of CNT assemblies. We predict this may be of greatsignificance in future attempts to promote the applicability of CNTassemblies.

Acknowledgments

This research was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF) fundedby the Ministry of Education (NRF-2010-0023204)

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